The relative importance of four neurohumoral mechanisms involved in the daily regulation of arterial pressure are being examined in our laboratory: the renin-angiotensin system, vasopressin system, prostaglandin system, aldosterone and the low and high pressure stretch reflexes. The experiments differ from those being conducted in many other laboratories in that they are specifically designed to obtain quantitative data concerning the role of each system in the acute and long-term control of arterial pressure. (1) A series of studies on the renin-angiotensin aldosterone system are being performed to determine the quantitative importance of angiotensin versus aldosterone in hypertension and the extent to which salt intake can alter the hypertensive action the mechanism. The long-term significance of the renin-angiotensin system in pressure control is being evaluated by studying the effects of prolonged low-dose angiotensin infusions on the pressure sensitivity, the renal fluid-volume system, the aldosterone system, vasopressin-thirst system, and the baroreceptor reflexes. (2) A number of studies on the vasopressin system are designed to determine the acute open-loop feedback gain of the vasoconstrictor pressure control system and its long-term significance, since recent findings in conscious dogs indicate that the direct pressor activity may contribute to pressure control. (3) The role of the high pressure and low pressure stretch reflexes in modifying both acute and long-term changes in the hemodynamics and in the control of body fluids is being studied under a variety of circumstances. Normal and baroreceptor denervated dogs are being studied using computerized techniques that enable continuous collection and analysis of data over prolonged periods of time. The quantitative data obtained in all of the above studies will be evaluated by use of computer modeling techniques.